Blown tube collapse frame assembly

ABSTRACT

In a blown tube process of producing thermoplastic film or sheeting, the blown tube is collapsed by passing it through a trapezoidal frame assembly to change the circular cross section of the tube to a square cross section. The square cross section tube passing from the trapezoidal frame assembly is then passed through a predeterminedly controlled flattening transition section to a pair of nip rolls in a manner that the film arrives at the nip rolls with minimal stretching.

This application is a continuation in part application of copendingapplication Ser. No. 349,643 filed Apr. 9, 1973, now abandoned andassigned to the same assignee as the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved blown tube collapse frame assemblycomprising a trapezoid section to change the cross section of a circulartube of thermoplastic material passing therethrough to a square crosssection, and a transition section to pass the material from the squaresection to a flattened section between a pair of nip rolls with minimalmaterial stretching in the process.

2. DESCRIPTION OF THE PRIOR ART

The blown tube process of producing thermoplastic film is well known inthe prior art. One example of such a process is found in U.S. Pat. No.2,720,680, Gerow, wherein a thermoplastic material is melt extruded intubular form and the tubular form is retained by means of internal airpressure. The tube is passed between a pair of nip rolls to be woundinto strips or sheeting. As taught in the Gerow disclosure, the pressurewithin the tube may be increased to expand the tube or to biaxiallyorient the thermoplastic material of the tube or bubble.

One of the principal problems in the blown tube process relates to theapparatus utilized to collapse the bubble or tube from a circular crosssection to a flat cross section and from the flat section to atheoretical line between a pair of nip rolls, to be thereafter slit infilm strip or sheet form. A typical prior art mechanical arrangement toprovide for the collapse of a blown tube is illustrated in U.S. Pat. No.3,235,632, Lemmer. In the typical collapsing arrangement of Lemmer, theblown tube is caused to pass through a V shape or A frame system ofrollers which collapse the blown tube on two sides, i.e., twodimensional, from a circular cross section to a flat cross section. Thecollapse of such a tube section immediately from a circular crosssection to a flat cross section causes uneven stresses to be imposed onor set up in the film to cause stretching and bagging of the final filmwhich is most undesirable in many applications particularly for woundcapacitor roll dielectrics which require a significantly flat or layflat film.

SUMMARY OF THE INVENTION

The principal object of this invention is to provide a confining andconverging series of moving surfaces, rollers for example, which engagea blown tube passing therethrough to gradually change its circular crosssection to a square cross section, and from the square cross section thetube is confined in a precise geometric configuration to pass throughnip rolls.

It is another object of this invention to provide a trapezoidal frameincluding a first series of rollers in the sides thereof so that a blowntube passing through the frame is changed from a circular cross sectionto a square cross section, and an opposite trapezoidal transitionsection having a series of rollers in the sides thereof to flatten saidsquare section tube to a line section with minimal material stretching.

It is another object of the invention to provide a trapezoidal framewith a ladder-like series of rollers defining each wall or side thereofwhich engage and collapse a circular tube to one of a reduced and squarecross section and an opposite trapezoidal section with similar rollersto provide controlled flattening of the square section preceding theentry of the square cross section tube between a pair of nip rolls.

Briefly described, this invention in its preferred form comprises atrapezoid frame collapse assembly which is specifically adapted to beutilized in conjunction with the blown tube process of producingbiaxially oriented polypropylene film. The frame assembly is positionedjust prior to the nip rolls utilized to seal off the tube, and preparethe flattened tube for the following slitting operation. The frameassembly comprises four flat planar walls or sides, each side includinga ladder-like arrangement of rolls mounted with their axes transverse tothe axis of the frame and the path of the tube therethrough. The frameassembly gradually, and over an extended length of the tube transformsor converges the circular cross section of the tube at the frameentrance to one of a square cross section at the frame exit. At thispoint, a transition section comprising a four sided frame with aladder-like series of rollers in each side provides a controlleddivergent geometry flare out of the square section to a line sectionbetween the pair of nip rolls.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be better understood when taken in connection withthe following description and the drawings in which:

FIG. 1 is a schematic illustration of the frame assembly of thisinvention with a blown tube passing therethrough.

FIG. 2 is a schematic illustration of a prior art process of collapsinga circular tube to a flat cross section.

FIG. 2A is a side elevational view of FIG. 2.

FIG. 2B is an end view of FIG. 2.

FIG. 3 is a schematic illustration of the trapezoidal frame collapsing ablown tube of circular cross section to a square cross section and thento a flat cross section in accordance with this invention.

FIG. 3A is a side elevational view of FIG. 3.

FIG. 3B is an end view of FIG. 3.

FIG. 4 is a graph comparing the variation in path lengths between theflattening process of the circular tube and the square tube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is disclosed an inflated or blown tube 10which may be produced from any of the well known prior art processesincluding those as noted in the above patents. This blown tube 10 in thepresent invention is isotactic polypropylene and the tube diameter is onthe order of 50 inches. The tube 10 moves into a frame assembly 11 inthe direction as illustrated by the arrow. The frame assembly 11 ispositioned at the upper end of the tube, well away from the extrudeswhich forms the tube, and at a position after the tube is oriented orstretched. The frame assembly is described as adjacent the nip rolls.The frame assembly 11 in one preferred embodiment of this invention is aframe assembly of four flat planar sides 12, 13, 14 and 15, each sidebeing defined by a series of ladder-like arrangement of rolls 16 whichengage the surface of the tube in restraining fashion for passage of thetube through the frame assembly 11.

These rolls restrain and support the tube while the tube passestherethrough but do not specifically cause the tube to stretch or becompressed. The periphery of the square cross section emanating from theframe is substantially equivalent to the circumference of the tubeentering the frame assembly 11. It is understood that the square crosssection referred to includes as a practical matter slightly roundedcorners.

The number of rolls 16 for each side of the frame assembly is notparticularly critical. About 4 to 5 rolls of about 3 inches diameter foreach side, and extending over a frame height of about 12 feet issufficient for the tube material and size given. Also, the spacing ofthe rolls 16 is correlated to the tube material and its temperature andstretching characteristics so that the rolls are sufficiently close toeach other to prevent the tube from extruding or sagging therebetween.While these rolls 16 may in fact be driven rolls, in a preferredembodiment of the invention they are slave rolls or rolls which aredriven by frictional engagement with the passing tube 10. The tube 10 isdrawn through the frame 11 by other driven rolls such as nip or sealingrolls 17 positioned above the frame.

The frame assembly 11 may have all four sides parallel so as to form anunvarying channel or tunnel. However, it is preferred that the frameassembly 11 have a slight trapezoidal configuration so that each sideinclines toward the central axis to define an acute angle with thehorizontal of no more than about 6%, and preferably about 2° to 3°. Thisinclination provides a restraint to the tube 10 to insure a continuousand smooth transition from the round section 18 to the square section 19and to assure that the square section is formed into a stable squaresection at a precise location. Otherwise variations of the diameter oftube 10 may cause the square section to form at a transitory location orcause deformation of the square cross section, and either event maydisrupt the geometry of this invention. An abrupt change in the tubesection is also to be avoided as the tube in the location just prior tothe frame assembly 11 is still quite hot and could be adverselystretched by an abrupt transition from a round to a square section.

The blown tube 10 emanating from the trapezoidal frame assembly 11 isthen caused to pass between a pair of nip rolls 17 which carry off thetube in flattened cross section for the following slitting procedures.It is an important concept of this invention that the frame assembly 11provide a substantially square cross section tube 19 emanating from theend of the frame assembly 11. It is the square cross section 19 set upfor feeding into the nip rolls 17 which will provide the collapsing ofthe tube from the square cross section 19 to a line or flat crosssection 20 with minimal stretching and internal stressing of a film.However, the shape of the tube passing from a square cross section 19 toa line section 20 between nip rolls is critical if there is to beminimal stress and stretching of the tube material. In order to providethe correct geometry a flattening transition section 21 is provided onthe end of frame assembly 11 between the square section 19 and the niprolls 17.

Transition section 21 as disclosed in FIG. 1 comprises planar sets ofroller means on opposite sides of the square section 19 which channelthe square section 19 into progressively decreasing thicknessrectangular sections of increasing widths until the flattened section isreached. The overall configuration of the tube 10 in the transitionsection is a flared out wedge shape. In one form of this invention, aplanar set of roll means comprises oppositely and inwardly flaredbracket members 22 and 23. Each bracket member includes one or moreparallel rollers 24 mounted in ladder-like or planar relationship andpositioned to engage the tube and provide it with large flat planar sidesurfaces 25 and 26 extending from the square section 19 to the nip rolls17. A further set of opposite and outwardly flared bracket members 27and 28 each comprise one or more roller members 29 in planarrelationship which engage the opposite end surface 30 of the tube toform these surfaces into substantially planar surfaces. Some very smallarea of the tube between the upper roll 29 and the nip rolls 17 may beunsupported but the overall adverse result thereof is negligible.

An important criteria in the transition section is the acute angle θ₁(FIGS. 3 and 3A) between the plane of the brackets 22, 23 and 27, 28 andthe vertical. This angle θ₁ must be essentially equal for each bracket22 and 23, and 27 and 28. In the practice of the present invention, thetransition section is about 4 feet in height, and the noted angle θ₁ isabout 67.5° so that θ₂ is 22.5° or the total apex angle is 45°. Whilethis θ₁ angle may be less than 67.5° to about 65°, its increase is morebeneficient and θ₁ could increase to as much as about 75° with of coursea related change in θ₂. When the above geometries and relationships areutilized, a point on tube 10 at the square section 19 moves toward line20 between the nip rolls 17 along equidistant lines whose directions areultimately perpendicular to line 20. The direction and distance of theselines portray a non stretching condition in the tube material intransition section 21.

The improvement of this kind of collapsing apparatus and method overthose of the prior art is shown particularly with respect to the priorart FIGS. 2, 2A, and 2B. In FIG. 2, the tube 10 of circular crosssection is adapted to be flattened by passing through the pair of niprolls 17 so that it assumes the flat configuration of the line or axis19. FIG. 2A is a side elevational view of FIG. 2 and FIG. 2B is an endview of FIG. 2. The tube 10 is of a circular cross section at line 18and of a flat section at 19 between nip rolls 17. Now to FIG. 2B, thetube 10 becomes flattened at line 19 and in so doing each point on thetube 10 must take up a position on line 19. For example, a point A ontube 10 takes up the position A' on line 19. A point B takes up theposition B'. A point C on tube 10 takes up point C' on line 19 and etc.It is seen that in proceeding from points A through C and the like, eachcorresponding point on line 19 denotes a shorter path distance from thetube. These points on tube 10 must then travel different distances toreach the flattened configuration of line 19. These differences causethe internal stress and resulting bagginess of the film and areconsequently undesirable.

The improvement of the invention is now shown with respect to FIG. 3.Referring to FIG. 3, the tube configuration 10 is first changed from thecircular configuration 18 by means of the frame assembly 11 to thesquare cross section 20 as illustrated. After passing from the frame 11with its square cross section the tube is passed through a pair of niprolls 17 to be collapsed to the flat configuration of line 20.

Referring to FIG. 3B, it can be seen that during this flatteningprocess, a point A on the square cross section moves directly and moreperpendicularly to point A' on the flat cross section line 20. Similarlypoint B on the square cross section moves perpendicularly to the pointB' and so on. A point C on the square section moves also in a relativelystraight line to the point C' and then to C" on the flattenedconfiguration. Intermediate points between B and C and points followingC also move in this more perpendicular manner. Length of the paths ofany point are substantially equal, e.g., the path length AA' issubstantially equal to CC'C". Consequently the internal stresses in thefilm made by the process illustrated in FIG. 3 are greatly minimized sothat the film in the flat position retains its flatness with minimumbagginess set up by internal stresses. Any point on the square sectionmoves to the flattened section along relatively straight lines becauseof the rolls which are utilized at the flaring part of tube 10 betweenthe frame 11 and the nip rolls 17 as illustrated in FIG. 3.

A further comparison of the results of the use of the flatteningapparatus of FIG. 2 and the process and apparatus of FIG. 3 is shown inFIG. 4. In FIG. 4, the graph compares the percent deviation in pathlength in the two processes, i.e., the prior art process A and theprocess B of this invention.

These curves also take in to consideration the H/D ratio which is theratio of the vertical height of the frame 11 to the diameter of thetube. Rolls are preferred in this invention although it is understoodthat tube surface engaging means such as moving belts or caterpillararrangements may also be employed. The practice of this invention mayalso be accomplished by the well known air slide or air box process, orby slatlike surfaces which slide on and support the tube.

The collapse frame is designed to shape the bubble from a circularsection to a square section which is then collapsed into a nip. Theconventional frame configuration forces some portions of the bubble totravel longer distances to the nip than other portions. This rapidtransition results in stretching certain areas of the film. (Thegeometry of the round-to-square transition results in minimum pathlength deviation when the vertical angle between the tangent of thecircle and the mid-point of the square is 6° or less. The reason for theminimum deviation is that the sine and tangent functions are nearlyequal.) Theoretically, there is no path length deviation in thesquare-to-flat transition just prior to the nip rolls 17. The new frameassembly 11 is most feasible where the bubble remains constant, i.e.,where the bubble diameter entering frame assembly 11 remains constant.While some slight deviation from the square cross section can beaccommodated a significantly rectangular cross section is to be avoided.

While this invention has been disclosed with respect to particularembodiments thereof, numerous modifications may be made by those skilledin the art without departing from its true spirit and scope. Therefore,it is intended that the appended claims cover all such modifications andvariations which come within the true spirit and scope of the presentinvention.

I claim:
 1. In a blown tube apparatus in which an inflated circularcross section thermoplastic tube is collapsed from the circular sectionto a line or flat section, an improved collapsing frame assembly nextpreceding said flat section comprisinga. a frame assembly to surroundsaid tube, b. tube surface engaging means in said frame to engage saidtube over an extended length thereof to converge said cross section to asubstantially square cross section without significantly stretching orcompressing said tube material, c. a transition section on said frameassembly next adjacent said line section to collapse said square sectionto said line section1. said transition section comprising two pairs ofoppositely positioned and oppositely angled planar tube surface engagingmeans, the pairs of which are coincident with the parallel sides of saidsquare section and each planar set of tube surface engaging means havingan angle θ₁ between its plane and the longitudinal axis of the tubewhich are essentially equal so that points on said square section arriveat the line section along lines perpendicular thereto.
 2. The inventionas recited in claim 1 wherein said frame assembly comprises a convergenttrapezoidal frame through which said tube passes to change from acircular to a square cross section.
 3. The invention as recited in claim2 wherein the convergent angle of the said frame assembly is less thanabout 6°.
 4. The invention as recited in claim 3 wherein said equal andopposite angle θ₁ is from about 65° to about 75°.
 5. The invention asrecited in claim 3 wherein roller means are utilized for said tubeengaging means.
 6. The invention as recited in claim 4 wherein rollermeans are utilized in said tube surface engaging means of saidtransitional section.